1. Field of the Invention
The present invention relates to a permanent magnet and a method of producing the permanent magnet.
2. Description of Related Art
Among motors including brushless DC motor, a motor that is provided with a permanent magnet embedded rotor that has a rotor core and a plurality of permanent magnets that are embedded in the rotor core (which is hereinafter referred to as “IPM motor”) is well known. For example, the IPM motor is used as a driving motor for hybrid vehicles.
Making further reference to the permanent magnet that is used in actuators for motors or the like, neodymium magnets (also known as Nd—Fe—B sintered permanent magnets (such as Nd2Fe14B)) are finding wide application because of their excellent magnetic characteristics, and are used in vehicles including hybrid vehicles, which can be said to lead the current automobile industry; industrial machines, and wind power generators, which are attracting attention as clean energy sources.
Residual magnetization (residual magnetic flux density) and coercivity are often used as indices of magnet performance. For Nd—Fe—B sintered permanent magnets, the residual magnetization may be increased by increasing the volume fraction or improving the degree of crystal orientation, and the coercivity can be increased by refining the crystal grain size, using an alloy that has a high Nd concentration, or adding grains of a metal that has high coercivity.
The method that is most commonly used to improve the coercivity is to increase the anisotropic magnetic field of the metal compound by substituting a portion of Nd in an Nd—Fe—B alloy with a metal that has high coercivity, such as dysprosium (Dy) or terbium (Tb), in order to enhance the coercivity.
However, the amount of use of dysprosium or terbium as described above significantly exceeds the natural abundance ratio of rare-earth elements. In addition, because the estimated reserves in commercially developed ore deposits are very small and because the ore deposits are unevenly distributed around the world, the necessity of element strategy has been recognized. It is known that the abundance ratio of terbium is significantly smaller than that of dysprosium.
As described above, the coercivity of a permanent magnet may be improved by substituting some Nd with dysprosium or terbium, whereas it is known that the presence of the substitute causes a decrease in the saturation polarization of the permanent magnet. Thus, when the coercivity of a permanent magnet is increased by using dysprosium, a considerable decrease in its residual magnetic flux density should be allowed. In addition, because dysprosium and terbium are rare metals, it is needless to say that it is necessary to reduce the amount of use of dysprosium or terbium as much as possible from the viewpoint of resource risk and material cost.
The reason why dysprosium or the like that is diffused into grain boundaries of grain boundary phases cannot sufficiently diffuse into the deep interior of the grain boundary phases is described with reference to flow diagrams of FIGS. 9A to 9C that use a metallic structure diagram to show a conventional method of producing a permanent magnet.
As shown in FIG. 9A, in the internal structure of an Nd—Fe—B sintered permanent magnet that is composed of a main phase S and a grain boundary phase R, an Nd oxide OX1, such as Nd2O3 or NdOx, is present at triple points where three regions of the grain boundary phases R meet. When a layer of particles of a metal, such as dysprosium or terbium, is formed over the surfaces of the permanent magnet and a heat treatment is subsequently carried out, the metal diffuses along grain boundaries of the grain boundary phase R as shown in FIG. 9B (X direction).
If the dysprosium or the like that diffuses along the grain boundaries reaches the Nd oxide OX1 present at the triple points where regions of the grain boundary phase R meet, the dysprosium substitutes for the Nd in the oxide because dysprosium has a lower standard free energy of oxide formation than Nd and therefore combines more readily with oxygen than Nd. As a result, as shown in FIG. 9C, the Nd is expelled from the oxide and surrounds the oxide, whereas the dysprosium binds with oxygen to form a dysprosium oxide OX2 and cannot easily diffuse into the deep interior of the permanent magnet. To diffuse dysprosium into the deep interior, it is necessary to use a large amount of dysprosium, resulting in an increase in the material cost.
Japanese Patent Application Publication No. 2002-190404 (JP-A-2002-190404) describes a method for improving the magnetization by preliminarily introducing yttrium (Y), scandium (Sc) or lanthanum (La), which bonds more readily with oxygen than Nd, into the main phase of a magnet so that these metals can be expelled into the grain boundary phase to combine with oxygen during sintering in order to reduce the amount of Nd, which will be combined with and fixed by oxygen. Accordingly, 2000 ppm or more of oxygen is required in order to expel a larger amount of yttrium into the grain boundary phase.
When yttrium or the like is preliminarily present in the main phase of the permanent magnet described in JP-A-2002-190404, the magnetizing of the magnet may be greatly impaired if the yttrium or the like is not fully expelled from the main phase. In addition, because the coercivity of a permanent magnet may be improved by reducing the concentration of oxygen as much as possible and because some currently commercially available permanent magnets are produced in an atmosphere with an oxygen concentration of approximately 1000 ppm or lower, it is not preferred to produce a permanent magnet at an oxygen concentration of 2000 ppm.